A process which comprises grafting an ethylenically unsaturated silane compound onto an ethylene resin such as a polyethylene or a copolymer comprising ethylene in the presence of a radical initiator to prepare a modified ethylene resin and exposing the modified ethylene resin to water or steam in the presence of a silanol condensation catalyst to crosslink the resin, whereby physical properties such as heat resistance, creep resistance, environmental stress cracking resistance and resistance to chemicals can be markedly improved, is known from, for example, Japanese Patent Publication No. 1711/73. This crosslinking process has advantages over the conventional process which uses a chemical crosslinking agent or an electron beam in that the cost for equipment can be markedly reduced and the crosslinking operation is comparatively easy. Due to those industrial advantages, studies are being made actively to put this process into practical use in the fields of wire covering, pipes, films, and shrinkable tubes.
On the other hand, with respect to a propylene resin such as a polypropylene, it is predicted that excellent characteristics of heat resistance, high strength and high rigidity in addition to the same effects as in the case of the ethylene resin due to the excellent physical properties that the propylene resin inherently possesses can be obtained by crosslinking the propylene resin in the same manner as in the ethylene resin, and it is therefore expected that the expansion of the amount thereof and the applied fields would be very large.
In view of the above, the present inventors tried the above-mentioned crosslinking process for propylene resin. The resulting crosslinked product of the propylene resin was found to be greatly improved in heat resistance and other properties, but the impact resistance and rigidity were not sufficiently improved from the standpoint of practical use.
As a result of various investigations to overcome the above problems, it has been found that a crosslinked product of propylene resin having an excellent heat resistance and also practically sufficient performance in the impact strength and rigidity which are conflicting characteristics can be obtained by crosslinking a composition prepared by blending an ethylene resin and an ethylene-.alpha.-olefin copolymer rubber with a modified propylene resin with an ethylenically unsaturated silane compound grafted thereon.
Accordingly, an object of this invention is to provide a process for producing a crosslinked product of a propylene resin which comprises exposing a composition comprising (A) 60 to 90 wt % of a modified propylene resin with an ethylenically unsaturated silane compound grafted thereon, (B) 20 to 5 wt % of an ethylene resin, and (C) 20 to 5 wt % of an ethylene-.alpha.-olefin copolymer rubber, to water or steam in the presence of a silanol condensation catalyst to crosslink the composition.